Field of the Invention
The invention relates to a measuring configuration for measuring an electric variable, in particular a current intensity, on a high-voltage line by an electric transformer. The invention also relates to the use of such a measuring configuration.
A measuring configuration of this type is generally used for the purpose of monitoring the characteristic of the current intensity in a high-voltage system in order to be able to detect peaks in the current intensity in good time in the case of line damage. The measuring configuration is likewise used for the purpose of monitoring load-dependent currents. Furthermore, the measuring configuration is also used for the purpose of determining for high-voltage switching devices the correct disconnecting instant for interrupting the circuit.
In this case, the size and configuration of the individual components of the measuring device comply with the requirements set by the specific application or the specific site of application as regards mechanical or electric loading.
Usually, a measuring configuration for determining an electric variable on a high-voltage line such as, in particular, the current intensity, contains a container or tank which is at a high-voltage potential and through which the high-voltage line is guided by a continuous electric conductor. Disposed around the electric conductor in the interior of the tank is a usually annular electric transformer at which a measuring signal can be tapped. In order to insulate the transformer from the electric conductor and from the tank, the interior of the tank is filled with an insulating oil. In order to insulate the electric transformer further from the continuous conductor, the electric conductor can additionally have an insulating coating made from plastic or paper. The electric transformer itself is mostly configured as an electric coil, and in this case typically has a steel core around which a specific number of windings made from an insulated wire are wound. The voltage building up inductively in the windings is tapped from outside as a measuring signal via measuring terminals. Given a known number of windings, it is thereby possible to reach a conclusion as to the current intensity in the high-voltage line.
As a rule, the tank having the transformer and the continuous electric conductor is disposed at the height of the high-voltage line. For this purpose, the tank is disposed on an insulator or insulating body that insulates the tank from ground potential. The mechanical properties of the insulator must be selected in this case such that the insulator withstands the mechanical loads that arise from the weight of the measuring configuration. In particular, in areas at risk from earthquakes this requires high requirements to be set on the mechanical loadability of the insulator. The insulator is made from porcelain as a rule, for this reason.
The insulator itself is installed via a support configuration on the ground. The support configuration in this case has an evaluating device that evaluates the measuring signal of the electric transformer. The measuring terminals of the electric transformer are guided for this purpose through the insulator into the evaluating device. The measuring terminals are electrically insulated in this case from the tank of the transformer. Usually, maintaining or replacing the insulating oil is also undertaken via the support configuration.
Since the tank in which the electric transformer is disposed is at the same potential as the continuous electric conductor, and thus at the same potential as the high-voltage line, the term "live-tank measuring configuration" is also used for such a measuring device.
The measuring configuration described has been successfully used for many years in high-voltage technology. Nevertheless, such a measuring configuration disadvantageously harbors certain risks. Thus, for example, in the case of high mechanical loading, the porcelain insulator can burst and endanger the surroundings of the measuring configuration by flying fragments. Furthermore, ignition of the insulating oil, which may entail an explosion of the container of the measuring configuration, can occur in the case of a short circuit of the high-voltage line in the vicinity of the container or in the container itself, in particular between the continuous conductor and the electric transformer. Again, such an explosion endangers not only the measuring configuration itself, but also the immediate and close surroundings. Thus, both the insulating oil and the porcelain insulator harbors a certain safety risk in the measuring configuration of conventional construction.
For this reason, strict safety regulations must be followed when installing the measuring configuration and when maintaining and changing the insulating oil. Furthermore, when configuring the insulator it must be ensured that it also withstands mechanical loads that exceed the loading owing to the weight of the oil-filled container. Nevertheless, a porcelain insulator that is under a high compressive stress poses a relatively high risk. In particular, when the porcelain insulator is wantonly destroyed, it splits up explosively, with the result that the flying fragments can cause grave injuries.
In addition to the safety risks described for the measuring configuration of conventional construction, the measuring configuration is also relatively expensive because of the required size of the post insulator and because of the requirement for insulating oil. The maintenance of the measuring configuration is also time-consuming and expensive because of the safety regulations to be observed.
Even if, as currently known, the porcelain insulator of the measuring configuration is replaced by a so-called composite insulator which contains an inner tube made from a glass fiber reinforced plastic and sheds, applied thereto, made from a silicone rubber, this composite insulator must nevertheless be capable of bearing the load of the oil-filled container in the interior of which the electric transformer and the continuous electric conductor are disposed. For this reason, it is necessary in the case of a composite insulator as well for the diameter of the inner tube to be selected to be relatively large, and this in turn greatly increases the costs of production and material, since, in particular, the required quantity of expensive silicone rubber for the sheds is increased.